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        <h2 id="深入观察者与被观察者模式"><a href="#深入观察者与被观察者模式" class="headerlink" title="深入观察者与被观察者模式"></a>深入观察者与被观察者模式</h2><p>下面以一个经典的例子进行讲解:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//创建一个被观察者（开关）</span></span><br><span class="line"> Observable switcher=Observable.create(<span class="keyword">new</span> Observable.OnSubscribe&lt;String&gt;()&#123;</span><br><span class="line"></span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">call</span><span class="params">(Subscriber&lt;? <span class="keyword">super</span> String&gt; subscriber)</span> </span>&#123;</span><br><span class="line">            subscriber.onNext(<span class="string">"On"</span>);</span><br><span class="line">            subscriber.onNext(<span class="string">"Off"</span>);</span><br><span class="line">            subscriber.onNext(<span class="string">"On"</span>);</span><br><span class="line">            subscriber.onNext(<span class="string">"On"</span>);</span><br><span class="line">            subscriber.onCompleted();</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;);</span><br><span class="line"><span class="comment">//创建一个观察者（台灯）</span></span><br><span class="line"> Subscriber light=<span class="keyword">new</span> Subscriber&lt;String&gt;() &#123;</span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onCompleted</span><span class="params">()</span> </span>&#123;</span><br><span class="line">            <span class="comment">//被观察者的onCompleted()事件会走到这里;</span></span><br><span class="line">            Log.d(<span class="string">"DDDDDD"</span>,<span class="string">"结束观察...\n"</span>);</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onError</span><span class="params">(Throwable e)</span> </span>&#123;</span><br><span class="line">                <span class="comment">//出现错误会调用这个方法</span></span><br><span class="line">        &#125;</span><br><span class="line">        <span class="meta">@Override</span></span><br><span class="line">        <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">onNext</span><span class="params">(String s)</span> </span>&#123;</span><br><span class="line">            <span class="comment">//处理传过来的onNext事件</span></span><br><span class="line">            Log.d(<span class="string">"DDDDD"</span>,<span class="string">"handle this---"</span>+s)</span><br><span class="line">        &#125;</span><br><span class="line"><span class="comment">//订阅</span></span><br><span class="line">switcher.subscribe(light);</span><br></pre></td></tr></table></figure>

<p>针对上面的代码提出两个问题:</p>
<ol>
<li>被观察者中的Observable.OnSubscribe是什么，有什么用?</li>
<li>call(subscriber)方法中，subscriber哪里来的?</li>
<li>为什么只有在订阅之后，被观察者才会开始发送消息?</li>
</ol>
<h2 id="Observable-类深入"><a href="#Observable-类深入" class="headerlink" title="Observable 类深入"></a>Observable 类深入</h2><p>首先看一下OnSubscribe是什么?</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//前面也提到Acton1这个接口，内部只有一个待实现call()方法</span></span><br><span class="line"><span class="comment">//没啥特别，人畜无害</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">Action1</span>&lt;<span class="title">T</span>&gt; <span class="keyword">extends</span> <span class="title">Action</span> </span>&#123;</span><br><span class="line">    <span class="function"><span class="keyword">void</span> <span class="title">call</span><span class="params">(T t)</span></span>;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//OnSubscribe继承了这个Action1接口,自己并没有增加新的方法;</span></span><br><span class="line"><span class="comment">//但是新增了 Subscriber 类的泛型要求,也就是说只能用在订阅者中</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">OnSubscribe</span>&lt;<span class="title">T</span>&gt; <span class="keyword">extends</span> <span class="title">Action1</span>&lt;<span class="title">Subscriber</span>&lt;? <span class="title">super</span> <span class="title">T</span>&gt;&gt; </span>&#123;</span><br><span class="line">      <span class="comment">// OnSubscribe仍然是个接口</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>前面也提到<code>Acton1</code>这个接口，内部只有一个待实现<code>call()</code>方法.<br><code>OnSubscribe</code>接口继承了这个<code>Action1</code> 接口,但是自己并没有增加新的方法;只是把泛型限定在了 <code>Subscriber</code>,也就是说必须传入观察者对象;<br>那么也就是说，<code>OnSubscribe</code>本质上也是和<code>Action1</code>一样的接口，只不过它专门用于<code>Observable</code>内部。</p>
<p>那么将<code>OnSubscribe</code> 接口传入<code>Observable</code> 后,该接口干什么?</p>
<p>答:用来触发事件的发生,在<code>call()</code>内部调用观察者的 <code>onNext()</code>方法!!!</p>
<p>要注意的是在 <code>Observable</code> 被观察者的类中，<code>OnSubscribe</code>是它唯一的属性,<br>同时也是<code>Observable</code>构造函数中唯一必须传入的参数，也就是说，只要创建了<code>Observable</code>，那么内部也一定有一个 <code>OnSubscribe</code> 实体对象。</p>
<p>当然,<code>Observable</code>是没有办法直接new的,我们只能通过create(),just()等等方法创建,这些方法背后去调用了<code>new Observable(onSubscribe)</code></p>
<p>再来大体看一下 <code>Observable</code>这个类的骨架:</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Observable</span>&lt;<span class="title">T</span>&gt; </span>&#123;</span><br><span class="line">    <span class="comment">//唯一的属性</span></span><br><span class="line">    <span class="keyword">final</span> OnSubscribe&lt;T&gt; onSubscribe;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//构造函数，因为protected，我们只能使用create函数</span></span><br><span class="line">    <span class="function"><span class="keyword">protected</span> <span class="title">Observable</span><span class="params">(OnSubscribe&lt;T&gt; f)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.onSubscribe = f;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//create(onSubscribe) 内部调用构造函数。</span></span><br><span class="line">    <span class="keyword">public</span> <span class="keyword">static</span> &lt;T&gt; <span class="function">Observable&lt;T&gt; <span class="title">create</span><span class="params">(OnSubscribe&lt;T&gt; f)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">return</span> <span class="keyword">new</span> Observable&lt;T&gt;(RxJavaHooks.onCreate(f));</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="Observer-类深入"><a href="#Observer-类深入" class="headerlink" title="Observer 类深入"></a>Observer 类深入</h2><p>Observer是一个接口,其内部有三个方法:</p>
<ul>
<li>void onCompleted():处理被观察者发送完事件后的逻辑</li>
<li>void onError(Throwable e):处理出错逻辑</li>
<li>void onNext(T t):处理事件,有一个事件就执行一次</li>
</ul>
<p>顺便说一下 Subscriber,其实 Observer的实现类,但也是抽象的,主要是也实现了Subscription接口,所以相对于 Observer接口添加了一些新的方法,也有一些除了这两个接口之外自定义的方法,都自己实现了</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">abstract</span> <span class="class"><span class="keyword">class</span> <span class="title">Subscriber</span>&lt;<span class="title">T</span>&gt; <span class="keyword">implements</span> <span class="title">Observer</span>&lt;<span class="title">T</span>&gt;, <span class="title">Subscription</span></span></span><br></pre></td></tr></table></figure>

<p>看一下其中的主要方法:</p>
<p>Observer中的方法:(Observer中的方法在Subscriber中均未实现,需要开发者自己实现,只有这个三个Subscriber没有实现)</p>
<ul>
<li>void onCompleted():处理被观察者发送完事件后的逻辑</li>
<li>void onError(Throwable e):处理出错逻辑</li>
<li>void onNext(T t):处理事件,有一个事件就执行一次</li>
</ul>
<p>Subscription中的方法:</p>
<ul>
<li>void unsubscribe():取消订阅,停止接受被观察者发来的消息;</li>
<li>boolean isUnsubscribed():是否取消了订阅被观察者发的消息;</li>
</ul>
<p>Subscriber自身的方法:</p>
<ul>
<li><p>void add(Subscription s):添加一个订阅对象</p>
</li>
<li><p>void onStart():是一个空实现,开发者可以自己实现该方法,该方法被调用的时机是在被观察者与观察者产生订阅,但是此时被观察者还为发出任何消息</p>
</li>
</ul>
<h2 id="subscribe-subscriber-方法深入"><a href="#subscribe-subscriber-方法深入" class="headerlink" title="subscribe(subscriber)方法深入"></a>subscribe(subscriber)方法深入</h2><p>当创建了 Observable 和 Observer 之后，调用subscribe(subscriber)方法产生订阅关系时，发生了什么呢？</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//给被观察者传入了观察者对象</span></span><br><span class="line"><span class="comment">//这个方法是有返回值的,其类型是Subscription,熟悉吗?这是上面Subscriber对象实现的某一个接口;我们经常拿到这个引用,然后调用其unsubscribe()方法防止内存泄露啊!!!</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">final</span> Subscription <span class="title">subscribe</span><span class="params">(<span class="keyword">final</span> Observer&lt;? <span class="keyword">super</span> T&gt; observer)</span> </span>&#123;</span><br><span class="line">   ....</span><br><span class="line">    <span class="comment">//调用了下面的方法,将observer类型包装成subscribe类型</span></span><br><span class="line">    <span class="keyword">return</span> subscribe(<span class="keyword">new</span> ObserverSubscriber&lt;T&gt;(observer));</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">final</span> Subscription <span class="title">subscribe</span><span class="params">(Subscriber&lt;? <span class="keyword">super</span> T&gt; subscriber)</span> </span>&#123;</span><br><span class="line">    <span class="comment">//往下调用</span></span><br><span class="line">    <span class="keyword">return</span> Observable.subscribe(subscriber, <span class="keyword">this</span>);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//其实最终是调用了这个方法</span></span><br><span class="line"><span class="keyword">static</span> &lt;T&gt; <span class="function">Subscription <span class="title">subscribe</span><span class="params">(Subscriber&lt;? <span class="keyword">super</span> T&gt; subscriber, Observable&lt;T&gt; observable)</span> </span>&#123;</span><br><span class="line">   <span class="comment">//注:这个方法是在被观察者中的方法,执行订阅函数,那么此时被观察者就持有了观察者的引用了</span></span><br><span class="line">  <span class="comment">//也就是说现在被观察者可以调用观察者的任何方法了;</span></span><br><span class="line">  <span class="comment">//被观察者持有观察者的引用,这是整个观察者模型最关键的地方;</span></span><br><span class="line">  </span><br><span class="line">  </span><br><span class="line">    <span class="comment">//刚产生订阅关系的时候,这时被观察者还未发送消息,先执行观察者的onStart()方法</span></span><br><span class="line">    subscriber.onStart();</span><br><span class="line"></span><br><span class="line">    <span class="comment">// add a significant depth to already huge call stacks.</span></span><br><span class="line">    <span class="keyword">try</span> &#123;</span><br><span class="line">        <span class="comment">// 在这里简单讲，对onSubscribe进行封装，不必紧张。</span></span><br><span class="line">      <span class="comment">//该方法的源码单纯的将 onSubscribe 对象返回了,什么也没做,所以你可以忽略这一步;</span></span><br><span class="line">        OnSubscribe onSubscribe=RxJavaHooks.onObservableStart(observable, observable.onSubscribe);</span><br><span class="line"></span><br><span class="line">        <span class="comment">//这个才是重点！！！</span></span><br><span class="line">        <span class="comment">//这个调用的具体实现方法就是我们创建观察者时</span></span><br><span class="line">        <span class="comment">//写在Observable.create()中的call()呀</span></span><br><span class="line">        <span class="comment">//而调用了那个call(),就意味着事件开始发送了</span></span><br><span class="line">        onSubscribe.call(subscriber);</span><br><span class="line">        </span><br><span class="line">		<span class="comment">//源码中该方法的实现是:单纯的把subscriber返回了;</span></span><br><span class="line">      <span class="comment">//其实在真正使用时我们并不关心订阅函数的返回值,反正该方法中被观察者发送消息,观察者处理消息的逻辑已经执行了;</span></span><br><span class="line">        <span class="keyword">return</span> RxJavaHooks.onObservableReturn(subscriber);</span><br><span class="line">        &#125; <span class="keyword">catch</span> (Throwable e) &#123;</span><br><span class="line"></span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> Subscriptions.unsubscribed();</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p>代码看到这里，我们就可以对上面三个问题做统一的回答了：</p>
<ol>
<li>onSubscribe是Observable内部唯一属性，是连接Observable和subscriber的关键，相当于连接台灯和开关的那根电线,主要是其内部的call()方法,在该方法中发送了消息;</li>
<li>call(Subscriber&lt;? super String&gt; subscriber)中的subscriber，就是我们自己创建的那个观察者</li>
<li>只有在订阅的时候，才会发生onSubscribe.call(subscriber)，进而才会开始调用onNext(),onComplete()等。</li>
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